Monitoring stations in these locations do have their own datum - a height in metres fixed relative to mean sea level. But, because of their location, these stations usually have the same datum as actual mean sea level. So it's expressed as zero metres.

How often are tide levels in coastal and estuary areas measured?

We use measuring stations like this one to collect water level data, usually once or twice a day. We collect data more frequently
when there's a greater risk of flooding. That's why the graph might show a short time lag since the last
measurement.